Role of Gonadal Steroids and Inhibitory Photoperiod in Regulating Body Weight and Food Intake in Deer Mice ( Peromyscus maniculatus )

Abstract

Abstract

We investigated the role of declining daylength and gonadal steroids on body weight and food intake in male deer mice (Peromyscus maniculatus). This species was chosen for study because individual males display different reproductive responses to inhibitory daylength. About one-third of all mice exposed to short days undergo testicular regression and exhibit reduced circulating levels of luteinizing hormone and testosterone (reproductive responsive males). In contrast, testicular function and circulating levels of both these hormones remain unaffected in an equal number of mice (reproductive nonresponsive males). Previous studies have shown that each phenotype exhibits a distinct set of metabolic responses to short days, including adjustments in body weight. These characteristics make deer mice a useful animal model in which to study the interactive effects of gonadal steroids and photoperiod on neural substrates regulating body weight. A gonadectomy/steroid replacement experimental model was used to test the role of testosterone in regulating body weight and food intake in short day-housed male deer mice. Among gonad-intact males, short daylength caused a decline in body weight in both reproductive responsive and nonresponsive individuals. However, reproductive responsive mice lost significantly more body weight than did nonresponsive mice. Furthermore, while the weight loss was accompanied by a significant reduction in food intake in responsive mice, the relatively minor weight loss in nonresponsive mice was not accompanied by a change in food intake. Because changes in body weight and food intake (data not shown) occurred nearly simultaneously during the 8-week exposure to short day-length, results suggest that modifications in body weight are not responsible for the decline in food consumed, and vice versa. Gonadectomized reproductive responsive mice lost the same amount of weight as intact responsive mice but ate significantly more food. Among nonresponsive males, gonadectomy led to significantly greater weight loss, relative to intact mice, but caused an increase in food intake per gram body weight. Steroid replacement prevented weight loss and increased food intake in both gonadal phenotypes. Despite the observations that food intake was steroid dependent and the magnitude of the effect differed between reproductive phenotypes, changes in food consumption do not fully explain the inhibitory effects of short days on body weight in either phenotype. Taken together with previous studies, these results suggest that reproductive quiescence confers significant metabolic benefits to individual deer mice by reducing the amount of daily energy requirements via a reduction in body weight. Conversely, maintenance of reproductive function during the nonbreeding winter season carries greater metabolic costs; these costs lead to increased amounts of food required to maintain body weight.